huntington's disease, which causes degeneration of neurons, runs in mark's family. huntington's is a heritable, autosomal dominant genetic disease. in an autosomal dominant disease, only one abnormal allele is required to cause the disease in offspring. however, it is possible that not all offspring will receive the abnormal allele from an affected heterozygous parent. family members that have the disease include mark's maternal grandfather and mark's maternal aunts. mark's maternal grandmother does not have the disease